High-strength steel having aging properties

ABSTRACT

STEELS HAVING THE FOLLOWING COMPOSITION: .08 TO .18% CARBON; .3 TO 1.0% MANAGANESE; .01 TO .05% COLUMBIUM; .008 TO .014% NITROGEN; .10% MAXIMUM SILICON; LESS THAN A TOTAL OF APPROXIMATELY .02% OF THE NITRIDE-FORMING ELEMENTS ALUMINUM, ZIRCONIUM, VANADIUM AND TITANIUM; AND THE BALANCE ESSENTIALLY IRON, AFTER APPROPRIATE PROCESSING, HAVE YIELD STRENGTHS OF 50,000 TO 70,000 P.S.I. IN HOT-ROLLED AND COLD-ROLLED CONDITIONS AND YIELD STRENGTHS OF 70,000 TO 90,000 P.S.I. AFTER STRAINING AND AGING.

June 20, 1972 J BUCHER ETAL 3,671,334

HIGH-STRENGTH STEEL HAVING AGING PROPERTIES Filed Aug. '7, 1970 2Sheets-Sheet l 4 mvsmons JOHN H. BUCHER BY JOHN F. HELD heir ATTORNEYITEMPER REDUCTION N w 0 I 4 T C C U w )D m m 13 H E mu 9 9 T M 7 D M z v8 L 8 L 3 3 o n n N 3 w C 6 6 n a m a t T O O IIC 5 5 U D h I. R N E N gP g m 0 0| d moIl- C n C T F PM W R E w M 13 mm N 7 W 8 W 0 WM 3 3 3 C 33 3 2 3 0 3 c 6 6 6 w 6 a 6 a 4 ll 0 6 r o O O 5 o 0 o 5 5 O 9 8 8 7 2 l0 7 7 6 w 5 w m. l. mm O IFQZMIFW ZOE.. 0ZO m Bt OTIFOZMKkW 2023020 5June 20, 1972 J. H. BUCHER ETAL 3,671,334

HIGH-STRENGTH STEEL HAVING AGING PROPERTIES Filed Aug. 7. 1970 2Sheets-Sheet 2 INVENTORS JOHN H. BUCHER JOHN F. HELD theirATTORNEYTEMPER REDUCTION United States Patent 3,671,334 HIGH-STRENGTH STEELHAVING AGING PROPERTIES John H. Bucher, Bethel Park, and John F. Held,Verona,

Pa., assignors to Jones 8: Laughlin Steel Corporation,

Pittsburgh, Pa.

Filed Aug. 7, 1970, Ser. No. 62,073 Int. Cl. C21d 7/14; C22c 39/54 US.Cl. 148-123: 14 Claims ABSTRACT OF THE DISCLOSURE and cold-rolledconditions and yield strengths of 70,000 to 90,000 p.s.i. afterstraining and aging.

This invention relates to renitrogenized columbiummodified high-strengthsteels, strain-aged articles manufactured from such steels and processesfor producing the articles.

We have developed steels which have yield strengths of 50,000 to 70,000p.s.i. in hot-rolled and cold-rolled conditions and yield strengths of70,000 to 90,000 p.s.i. in a strained and aged condition. The steelshave the following composition: .8 to .18% carbon; .3 to 1.0% manganese;.01 to .05% columbium; .008 to .014% nitrogen; .10% maximum silicon;less than a total of approximately .02% of the nitride-forming elementsaluminum, zirconium, vanadium, and titanium; and the balance essentiallyiron.

The steels of our invention have particular utility where there is arequirement for a medium-carbon steel capable of being formed intoarticles which after forming and aging develop high strengths. Thus, ina hot-rolled condition, the steels of the invention are formed intoreinforcing members for commercial and passenger vehicles and in acold-rolled condition are formed into pillars, crossmembers, seattie-downs and floor pans for such vehicles. In a particular application,the steels of our invention are formed, painted, and baked and dependingon their exact chemistry and processing history have yield strengths of50,000 to 70,000 p.s.i. before forming and develop a flow stress of70,000 to 90,000 p.s.i. after forming (straining) and baking (aging).

The amount of reduction the product is subjected to during cold-rollinginfluences the aged yield strength of the formed article; higher coldreductions are reflected in higher strengths. In addition, temperrolling of both the hot-rolled and cold-rolled product results in higheraged yield strengths. Also the steels of the invention in a coldrolledcondition when subjected to a continuous annealing procedure, e.g.,normalizing, develop higher aged yield strengths than when batchannealed.

Columbium is, of course, the major contributor to the high strengths ofthe steels of the invention and the nitrogen is primarily responsiblefor the increased strengths developed by the steels upon aging. Theprimary effect of columbium in imparting strength to the steels is torefine the grain size of the steels. Significantly, columbium is theonly one of the conventionally employed grain refining elements, i.e.,zirconium, vanadium and titanium, which is not also a strong nitrideformer; and; consequently, is the only one of these elements which canbe used in conjunction with nitrogen to produce a strain- 3,671,334Patented June 20, 1972 aging steel. Correlatively, the zirconium,vanadium, and titanium levels in the steels of the invention are eachkept below about .005% so that the formation of nitrides is kept at aminimum. In addition, aluminum, which is also a strong nitride former ismaintained at a level below about .005%.

An object of the present invention is to provide' mediumcarbon steelshaving yield strengths of 50,000 to 70,000 p.s.i. in a hot-rolled orcold-rolled condition and yield strengths of 70,000 to 90,000 p.s.i.after straining and agmg.

Another object of the invention is to provide such steels in the form ofcolumbium-modified renitrogenized steels.

Yet another object of the invention is to provide strain aged articlesmanufactured from such steels.

Still another object of the invention is to provide proccesses forproducing such articles from such steels.

These and other objects and advantages of the present invention will beevident from the following detailed description of the invention withreference to the accompanying drawing in which:

FIG. 1 is a series of curves illustrating the variation in I the agedyield strength and percent total elongation with percent temperreduction of steels of the invention in a hot-rolled condition.

FIG. 2 is a series of curves illustrating the effects that variations inthe amount of cold reduction have on the physical properties of a steelof the invention in the strained and aged condition.

FIG. 3 is a series of curves illustrating the effects of a batchannealing operation and a continuous annealing operation on the physicalproperties of a steel of the invention in the strained and agedcondition.

Two steel heats (identified herein as #63387 and #72319) having thecompositions set out in the table were produced according toconventional steelmaking practices.

The high nitrogen levels were obtained by making calcium cyanamide ladleadditions to the steels. The manned in which nitrogen is added is notcritical, however, and other techniques such as the use of high-nitrogenferromanganese can be employed to renitrogenize the steels. The steelswere teemed and processed into hot-rolled strip by conventionalprocessing techniques. Hot-rolled stri-p manufactured from Heat #63387and Heat #72319 are referred to herein as #63387(H) and #72319(H)respectively. The #63387(H) strip had a yield strength of about 64,000p.s.i., and the #72319(H) strip had a yield strength of about 60,000p.s.i.

To determine the strain-aging properties of the hotrolled strip, samplesthereof were pulled on a tensiontesting machine so as to develop astrain of 7 /2% in the samples. Thereafter, the samples were heated at212 F. for 2 hours, cooled to room temperatures and their physicalproperties determined. This techniques provides an indication of theaged properties which are established in the product upon undergoing aconventional forming operation, such as the forming of reinforcingmembers for passenger vehicles, and the same or an equivalent heattreatment. Samples were also temper rolled before being strained toascertain the eflect of temper rolling on the strain-aged properties ofthe product. The results of the tests are shown in FIG. 1.

Both the #63387(H) product and the #72319(H) product developed an agedyield strength of about 78,000 p.s.i. in the absence of a temperreduction. However, the aged yield strength of the #63387 (H) productincreased rapidly with the increased percentage temper reduction,reaching a maximum of about 88,000 p.s.i. at a temper reduction of about2%, whereas the aged yield strength of the #72319-(H) product increasedto a maximum of about 86,000 p.s.i. at a temper reduction of about 2%The percentage total elongation for each product fell as the percenttemper reduction increased, indicating the need to keep temper reductionat a minimum where maximum ductility is sought.

To determine the suitability of the steels for cold-rolled applications,samples of the #63387(H) product were cold reduced 50% and 64% byrolling. The cold-reduced material is referred to herein as #63387(C).The #63387(C) samples were batch annealed for 10 hours at 1250 F., the64% cold-reduced material possessing a yield strength of 53,000 p.s.i.and the 50% cold-reduced material possessing a yield strength of 50,000p.s.i. upon completion of the annealing cycle. As with the hotrolledstrip, the annealed cold-rolled samples were temper rolled variousamounts, pre-strained 7 /2%, and aged by heating at 212 F. for 2 hours.The samples were then tested and the results are presented in FIG. 2,the left-hand side of the figure indicating the results for the 64%cold-reduced product and the right-hand side of the figure indicatingthe results for the 5 0% cold-reduced product.

As with the hot-rolled strip, the aged yield strength of the cold-rolledproduct increased with increased temper reductions, reaching a value of70,000 p.s.i. at about a 1% temper reduction for the 64% cold-rolledproduct and at about a 3% temper reduction for the 50% cold rolledproduct. The greater strength of the 64% coldrolled product reflects itssmaller annealed grain size. The bottom half of FIG. 2, illustrating thepercentage total elongation values of the samples, indicates the need,as with the hot-rolled strip, to keep temper reductions at a minimum toobtain optimum ductility.

The steels of the invention demonstrate a more pronounced aging effectwhen they are continuously annealed rather than batch annealed afterundergoing cold reduction. This is shown in FIG. 3 which presentsresults for samples of #72139(H) product which were cold rolled 55%,annealed for 6 hours at 1210 F. or normalized at 1585 F. for 2 minutes,prestrained 7 /2%, and heated at 212 F. for 2 hours. The samples werealso temper rolled various amounts. The samples annealed for 6 hours at1210 F. possessed a yield strength of about 49,000 p.s.i. Upon strainingand aging, the yield strength increased to about 60,000 p.s.i., as shownat the right-hand side of FIG. 3. The samples normalized at 1585 F. for2 minutes possessed a yield strength of 53,000 p.s.i., and uponstraining and aging, the yield strength increased to about 68,000 p.s.i.With both annealing procedures, the aged yield strengths of the samplesincreased with increased temper reductions, although the yield strengthsof the normalized samples were consistently about 8,000 p.s.i. greaterthan the batch-annealed samples. As with the previous samples,percentage total elongation decreased with increased percentage temperreduction.

In addition to a normalizing treatment, the continuousannealingoperation can be elfected by the heat treatment given steel strip on aconventional Sendzimer galvanizing line. A continuous-annealingoperation which provides for rapid cooling of the steel increases theamount of carbon in solution in the steel, rendering it susceptible tocarbon strain-aging. This effect is additive to the aging produced bythe nitrogen in solution and increases the total strain-aging potentialof the steel.

Applying the teachings set out herein, steels having yield strengths ofbetween 50,000 to 70,000 p.s.i. in a hot-rolled or cold-rolled andannealed condition and of between 70,000 to 90,000 p.s.i. afterstrain-aging can be produced from steels of the following composition:carbon, .08 to .18%; manganese, .3 to 1.0%; columbium, .01 to .05%;nitrogen, .008 to .014%; silicon, less than .10%; phosphorus and siliconin normal amounts; and less than a total of about .020% of aluminum,zirconium, vanadium and titanium. However, greater latitude inprocessing is obtained by the use of steels having the followingpreferred composition: carbon, .11 to .13%; manganese, .60 to .80%;columbium, .02 to .03%; and nitrogen, .012 to .014%.

The steels of the invention can be strain-aged over a wide range oftemperatures and time periods, and the particular application to whichthe steels are put primarily determines the particular conditions to beused. Generally, the steel after being formed into some commercialarticle will be heated at a temperature between 200 and 600 F. for atime period of between 5 to minutes to increase its strength level.

We claim:

1. A formed and strain-aged steel article, consisting essentially ofiron, 0.8 to .18% carbon, .3 to 1.0% manganese, .01 to .05 columbium,.008 to 0.14% nitrogen, .10% maximum silicon, and containing less than atotal of approximately .02% of the nitride-forming elements aluminum,zirconium, vanadium and titanium, there having been sufficient freenitrogen in the steel to impart strain aging properties thereto, saidarticle having a yield strength in excess of about 70,000 p.s.i.

2. The article of claim 1 wherein the carbon content is .11 to .13%, themanganese content is .60 to .80%, the columbium content is .02 to .03%and the nitrogen content is .012 to .0l4%.

3. A process for producing a steel article having a yield strength ofabove about 70,000 p.s.i. in a formed and strain-aged conditioncomprising:

(a) providing a steel consisting essentially of iron, 0.8 to .18%carbon, .3 to 1.0% manganese, .01 to 0.5% columbium, .008 to .14%nitrogen, .10% maximum silicon, and containing less than a total ofapproximately .02% of the nitride-forming elements aluminum, zirconium,vanadium and titanium, there being sufficient free nitrogen in the steelto impart strain aging properties thereto,

(b) hot-rolling the steel,

(c) forming the hot-rolled product into said article,

and

(d) heating the article at a temperature between 200 and 600 F. for 5 to120 minutes.

4. The process of claim 3 including temper rolling the hot-rolledproduct before forming the article. I

5. The process of claim 3 wherein the carbon content of the steel is .11to .13%, the manganese content is .60 to .80%, the columbium content is.02 to .03% and the nitrogen content is .012 to 014%.

6. The process of claim 5 including temper rolling the hot-rolledproduct before forming the article.

7. A process for producing a steel article having a yield strength ofabove about 70,000 p.s.i. in a formed and strain-aged conditioncomprising:

(a) providing a steel consisting essentially of iron, .08 to .18%carbon, .3 to 1.0% manganese, .01 to .05% columbium, .008 to 014%nitrogen, .10% maximum silicon, and containing less than a total ofapproximately .02% of the nitride-forming elements aluminum, zirconium,vanadium and titanium, there being sufficient free nitrogen in the steelto impart strain aging properties thereto,

(b) hot-rolling the steel,

(c) cold-rolling the hot-rolled product,

((1) annealing the cold-rolled product to restore its ductility,

(e) forming the annealed product into said article, and

(f) heating the article at a temperature between 200 and 600 F. for 5 to120 minutes.

8. The process of claim 7 wherein step (d) is carried out as acontinuous operation.

9. The process of claim 7 including temper rolling the annealed productbefore forming the article.

10. The process of claim 9 wherein step (d) is carried out as acontinuous operation.

11. The process of claim 7 wherein the carbon content of the steel is.11 to .13%, the manganese content is .60 to .80%, the columbium contentis .02 to .03% and the nitrogen content is .012 to .014%

12. The process of claim 11 wherein step (d) is carried out as acontinuous operation.

13. The process of claim 11 including temper rolling the annealedproduct before forming the article.

14. The process of claim 13 wherein step (d) is carried out as acontinuous operation.

References Cited UNITED STATES PATENTS 2,999,749 9/1961 Saunders et a1.75-58 3,010,822 1-1/ 1961 Altenburger et a1. 75-123 1 3,102,831 9/1963Tisdale 148-12 3,247,946 4/ 1966 Klein 148-123 3,254,991 6/1966 Shimminet al. 75-123 I X 3,402,080 9/1968 Kubota et al. 148-36 3,544,393 12/1970 Zanetti 14812 3,558,370 1/1971 Boni 148-36 3,155,549 11/1964Nakamura 75-124 X 3,180,726 4/ 1965 Nakamura 148-42 X 3,328,211 6/1967Nakamura 148-12 3,432,368 3/1969 Na'kamura 148-12 FOREIGN PATENTS685,397 4/1964 Canada 75-123 I 1,101,193 1/ 1968 Great Britain 75-123 Jr Small Niobium Additions on the Mechanical Properties of CommercialMild Steels, Journal of Iron and Steel Institute, January 1963, vol.201, pp. 43-46.

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant ExaminerU.S.. Cl. X.R.

75-123 B, 123 H, 123 J, 123 M, 124; 148-36

